This invention relates to semiconductor devices comprising one or more gate-controlled unipolar hot-carrier transistors, particularly but not exclusively for fast logic, fast switching or very high frequency amplification applications, which transistors can have a fast response time with strong control by a small gate signal. The invention further relates to methods of manufacturing such semiconductor devices.
The paper entitled "A New Majority-Carrier Diode--The Camel Diode" by Dr. J. M. Shannon, published in the Japanese Journal of Applied Physics, Vol. 19, Supplement No. 1 (1980), pages 301 to 304, discloses a semiconductor device comprising a semiconductor body including first and second semiconductor regions of one conductivity type, and a semiconductor barrier region located between the first and second regions, said barrier region having a net doping concentration of the opposite conductivity type and being sufficiently thin such that the depletion layers formed at zero bias with both the first and second regions merge together substantially to deplete said barrier region of mobile charge carriers. Current flow between the first and second regions during operation of the device is at least partially by thermionic emission of charge carriers of said one conductivity type across said barrier region.
The paper describes good quality bulk unipolar diodes having a very thin highly-doped p-type barrier region which forms a potential hump in the bulk of the semiconductor body, between an overlying very highly-doped n-type first region and a low-doped n-type substrate which forms the second region. The current flow through the diode occurs between the vertically-separated first and second regions and so is substantially perpendicular to the major surface of the body. The effect of the doping concentrations on the barrier height is described in said paper, and an approximate theoretical relationship is also given.
Compared with a metal-semiconductor Schottky diode, such a camel diode has an advantage in that the diode barrier is formed in the semiconductor bulk, and hence away from the semiconductor body surface. The doping level in the barrier region is sufficiently high such that the energy-band bending around the potential maximum is about or greater than k.T (where k is Boltzmann's constant and T is the absolute temperature) over a distance comparable with the charge-carrier mean-free path. Under these circumstances the current flow appears to be dominated by thermionic emission of majority carriers (i.e. charge carriers of said one conductivity type) over the potential barrier, and the current-voltage relationship is similar to that for a Schottky barrier having the same effective barrier height .phi.. Thus the current flow over the barrier varies exponentially with the barrier height, the exponent depending on the quality of the diode.
Several different camel diode structures are described in U.S. Pat. No. 4,149,174, and the use of such depleted barrier regions for the base-collector barriers of hot-carrier transistors is also described. The present invention provides a quite different hot-carrier transistor structure in which the barrier height of the barrier region is controlled to provide transistor characteristics.